With circuit arrangements of this type for detecting the synchronizing pulses, these pulses are mostly detected because those signal portions of the television signal which exceed a cut-off level in the direction towards the peak level are separated in a separating stage. The separated signal portions then represent in a way the synchronizing pulses. However, this procedure is beset with significant errors when the television signal is disturbed. If the synchronizing pulses do not always have an adequate amplitude i.e., which exceeds the cut-off level, then no synchronizing pulses are separated. A further problem is that the cut-off level often does not have the correct value and thus faulty or no synchronizing pulses are detected.
In the majority of prior art circuit arrangements the cutoff level is recovered in that, at the instants in which synchronizing pulses are expected, which is, for example, determined by a subsequent phase control circuit, the level of the television signal is determined and subsequently the cut-off level is set to a slightly lower level. This procedure functions, however, only then when the intervals in which the level of the television signal is determined accurately coincides with the synchronizing pulses. In the event of a desynchronization, however little, of the subsequent phase control loop the level of the television signal is then, for example, determined in the region of the porch of the signal. This immediately results in an erroneous cut-off level.
The European Patent Application No. 0,244,239 discloses a separating stage for synchronizing pulses in which the edges of the synchronizing pulses are detected. The signal then recovered is utilised to generate the cut-off level. However, this requires fixed assumptions about the edge steepness of the synchronizing pulses. Only when these boundary conditions are present can the edges of the pulses be detected. If, however, the criteria for the edges are not reached anymore, for example in the case of a disturbed television signal, an erroneous signal portion of the television signal may be detected as a synchronizing pulse. This then results in an erroneous fixing of the cut-off level with the above-described consequences.